The present invention relates generally to sonar equipment for underwater generation of sound and more particularly to an electromagnetic sound transducer or projector including pressure compensation means. More specifically the invention is directed toward a soar projector having piston means for generating acoustic waves capable of operation at lower frequencies.
In the past, electrodynamic projectors were used principally as low frequency and broadband laboratory calibration units. They were notoriously large, inefficient and were unreliable in all but laboratory environments; hence they were not viable candidates for field applications. The recent introduction of rare earth magnetic materials has drastically changed the electrodynamic transduction concept in critical areas and has resulted in a new low frequency transducer design disclosed herein.
In 1980 a new material called Samarium Cobalt was introduced. The raw materials were costly. Samarium cobalt demonstrated energy products on the order of 18 MGOe and extremely high coercive force, which renders the material practically nondemagnetizable. In 1985, Neodymium-Iron-Boron, was introduced. Energy products of 35 MGOe are presently available. Coercive force is also extremely high.
It is known that all permanent magnet materials lose their magnetism at elevated temperatures. The Curie temperature is the point at which a material is no longer magnetic. In samarium cobalt this occurs at 600.degree.-800.degree. C. (1100.degree.-1500.degree. F.); in neodymium-iron-boron, at 310.degree. C. (590.degree. F.). Irreversible losses may also occur at lower temperatures. Guidelines recommend the following operating temperature limits for the two materials: samarium cobalt +250.degree. C. (+480.degree. F.), neodymium-iron-boron +150.degree. C. (+300.degree. F.).
To conserve the force available in previous moving-coil units, pistons were made as light as possible. Lightweight radiators, for a given amount of transducer compliance, exhibit relatively high resonant frequencies. Because moving coil transducers exhibit flat responses above their fundamental resonant frequency it is desireable to make that frequency as low as possible. Resonant frequency is proportional to the inverse square root of the product of piston suspension compliance and piston acoustic mass. Compliance at moderate to extreme depths is controlled by the volume of the air cavity (compliance chamber) contained within the transducer. Reduction of resonant frequency can be achieved by increasing the compliance chamber volume. This can easily control overall transducer volume at only moderately low frequencies. In a similar manner, resonant frequency can be reduced through an increase in effective piston mass. This increase can be effected with no appreciable increase in overall transducer volume, even at extremely low frequencies. Because transducer volume, rather than weight, is usually the constrained physical parameter in shipboard applications, the mass-loading approach is appropriate. Implementation of mass-loading prior to availability of new magnetic materials would have prohibitively affected transducer size and weight because of the enclosed magnet volume and iron pole piece volume necessary to produce enough force to drive the mass load.
However, despite the aforementioned considerations, it is nevertheless disadvantageous to attempt to reduce the resonant frequency of the projector by increasing the mass of the piston means by actually adding to the size and/or weight thereof. Such additional weight not only increases the overall weight of the device, which is undesirable, but it also increases the loading of the projector thereby requiring increased power supply. to reduce the resonant frequency of a sonar projector by increasing the mass loading of the piston means without actually increasing the weight or size thereof.